This invention derives from a desire to prepare a blend of poly(vinyl chloride) (PVC) with an economical, available polymer; more particularly, to prepare a blend of PVC with a polyolefin (PO) which blend will benefit from fiber reinforcement to yield a composite which maximizes the beneficial properties and minimizes the drawbacks of the reinforced component polymers. To this end, the general approach has been to formulate a polyblend of the components which has particular superior properties compared to that of each component, then reinforce the superior blend with an appropriately sized glass fiber to produce a superior composite. Since PVC and PO are well known to be incompatible it was necessary to provide an appropriate compatibilizer which would enhance the properties of both PVC and PO.
Various methods are known to produce blends of normally incompatible polymers among which methods are grafting techniques and the use of a compatibilizer. The recognized function of the optimum compatibilizer is to produce a thermodynamically compatible blend in which the components are mutually miscible as is evidenced by a single glass transition temperature (Tg). More recently, as disclosed in U.S. Pat. No. 4,590,241 to Hohlfeld, miscible blends have been provided by blending a mixture of compatibilizing agents containing coreactive groups. The compatibilizing agents, by themselves, are normally incompatible. The emphasis in the '241 teachings is on the concept of obtaining compatibility or miscibility of the polymer components so as to obtain better notched impact, tensile strength, and other physical properties in the polyblend which then can be conventionally reinforced, if it is desired to do so.
With particular regard to heat distortion temperature (HDT) of a blend, HDT is believed to be ratioable from the individual HDTs of the components of the blend, but the accuracy of such an approximation is progressively diminished as the blend becomes less compatible. Addition of a rubbery component to the blend makes predicting the blend's properties even more speculative. The HDT of my unreinforced polyblend is of particular interest because it is lower than that of PVC, the blend's major component relative to the other components, and the blend would elicit little interest except that it possesses surprisingly good processability and solvent resistance. Of even greater interest is the discovery that, when reinforced with inorganic fibers, and specifically glass fibers, the reinforced blend shows an improvement in HDT which cannot be accounted for with knowledge of the improvement in HDT due to glass fiber, in glass fiber reinforced (GFR) PVC.
The blend of my invention has several distinct phases. The first is a thermodynamically compatible blend of polyolefin (PO) and grafted polyolefin (PO-G) which, because its components are crystalline, forms a single phase having a single Tg. The second phase consists essentially of amorphous poly(vinyl chloride) (PVC); by "amorphous" I refer to about 10% crystallinity, or less. The third phase consists essentially of a mixture of amorphous chlorinated polyethylene (a-CPE) and crystalline CPE (c-CPE), and optionally, an impact modifier such as ethylene-propylene diene monomer (EPDM). The second phase and third phases are mechanically compatible and exhibit two Tgs. When the first phase of olefin polymer (PO/PO-G) is blended with the PVC and CPE phases, and optionally, EPDM, the reactive melt alloying of the components of the blend is insufficient to give it miscibility as well as chemical compatibility. By chemical compatibility I refer to a blend in which there is a chemical interaction, and preferably a chemical coupling between the components. The blend may exhibit a single Tg, or more than one. For example, polyblends compatibilized with reactive polymers yield chemically compatible blends. Another example is a blend of polyolefin and polystyrene with a styrene/butadiene/styrene block copolymer as taught in U.S. Pat. No. 4,386,187.
It was hoped that the graft polyolefin (PO-G) in my blend would provide it with chemical compatibility which derived from reaction of the grafted chains with sites on the PVC chain. As will be shown hereinbelow, the reaction of such grafted chains is insufficient to provide the blend with chemical compatibility. My two-phase blend has only mechanical compatibility. Since chemical compatibility, if not miscibility, is typically an essential prerequisite of a desirable blend, my blend would have been ignored, except that, within defined ranges, this mechanical compatibility is fortuitously sufficient to provide the blend with substantial rigidity when the PVC is the major component relative to each other ingredient; also, excellent solvent resistance and processing characteristics, and good low temperature impact strength.
When the blend is reinforced with inorganic fibers, the inorganic fiber reinforced (IFR) articles have surprisingly high HDT and excellent physical properties, compared with that of GFR PVC. As applied to the polyblend of my invention, the term "compatible" refers to mechanical compatibility sufficient to avoid gross phase separation under extension, and the term "miscible" refers to thermodynamic compatibility indicative of a single phase. The distinction between mechanical and chemical compatibility is made by examining the results of a test in which the soluble components are washed out, also referred to as a "solvent uptake" test, as explained hereinafter.
Among the low-cost thermoforming plastic resins, PVC is the most favored for use in pipe, pipe fittings, and assorted construction uses, usually as impact-modified PVC, because it is easy to process. Its long-standing resistance to being successfully reinforced with glass fibers was recently overcome by the choice of a particular sizing on the glass, more fully disclosed in U.S.Pat. No. 4,536,360 to Rahrig, the disclosure of which is incorporated by reference thereto as if fully set forth herein. The processability of IFR PVC, and particularly GFR PVC containing more than 20 phr fibers (parts per 100 parts PVC resin) is notably diminished as the viscosity of the mixture increases.
It was therefore desirable to lower the viscosity of an IFR blend containing at least 20 phr (parts per 100 parts of all resin) inorganic fibers, yet maintain a concentration of at least 40 parts PVC in a polyblend which is to have better impact strength and a higher HDT than one might expect because of the PVC present. It was also desirable to have a polyblend which could be reinforced with sized glass fibers not limited to those disclosed by Rahrig, supra, yet which would fail in cohesive failure. By "cohesive failure" I refer to failure of a sample of GFR blend due to tearing of blend from blend, rather than, tearing of blend from the glass surface ("adhesive failure"). Thus, cohesive failure is predicated upon the blend's properties rather than the bond between the blend and surface of the glass fiber.
Such an IFR polyblend, reinforced with long inorganic fibers, would be expected to be thermoformable over a wide range of temperature, yet not sag when moved to a thermoforming station after being brought near the desired temperature. However, the relatively amorphous nature of rigid PVC, and the broad range over which it melts, dictates that the GFR thermoformed product would not only stay relatively soft and therefore unstable in shape, but also sticky for an uncomfortably long period. To counter these drawbacks it is conventional to add stabilizers, fillers and impact modifiers which would desirably still be effective in a polyblend of PVC. Thus, it was hoped that the proper choice of polyblend components, if found to resist gross phase separation, would lend itself to manipulation with known modifiers, and yield a rigid, hard and stable GFR thermoformed product which could be handled and stacked without sticking, soon after thermoforming.
In contrast with PVC, olefin polymers such as PP, PE or E-co-P are significantly more crystalline and have a relatively sharp melting point which does not allow a laminar sheet, even when reinforced with glass or other inorganic fibers, to support its own weight without sagging. Such polymers would therefore not be likely components of a PVC polyblend with the aforesaid desirable properties.
It is well known that PVC is incompatible with PP and PE (see Polymer Blends Vol I, edited by Paul & Newman, Academic Press 1978). This can be confirmed, in principle, if one calculates the compatibility of PVC and PP, or PVC and PE, or PVC and PP and PE, from two pieces of information for each polymer: solubility parameter, and, molecular weight. It is generally accepted that thermodynamic compatibility is the essential requirement of a blend with improved properties relative to those of its component polymers. It is therefore particularly unexpected that such improved properties are obtained with a thermoformable polyblend which is not a thermodynamically compatible blend of polymers, but only a mechanically compatible blend. The terms "blend" and "polyblend" are used interchangeably herein.
By thermodynamically "compatible" I refer to miscibility on a molecular scale, as evidenced by a single Tg which the polyblend of this invention does not exhibit. Neither does it exhibit gross symptoms of phase separation, referred to as being `cheesy`. What the blend does exhibit is mechanicial compatibility, resulting in highly desirable physical properties, though the HDT of the blend is not improved over that of its component polymers. Most important is the excellent processing characteristics of the blend, its smooth texture and non-cheesy consistency. These properties provide evidence of sufficient mixing of polymer segments on a microscopic scale to inculcate mechanical compatibility and a physical constraint so as to prevent demixing.
It is well known that CPE renders a blend of PVC and PO mechanically compatible, but a blend of 10 parts by wt CPE with 50 parts PVC and 50 parts PO results in a decrease in HDT. With the addition of the PO-G, mixing of polymer segments occurs resulting in further lowering of the HDT which can only be attributable to the combined presence of the CPE and PO-G which function as co-compatibilizers and which have substantially lower Tgs than PVC. This confirms that such melt alloying as does occur because of the presence of the PO-G, has little, if any, discernible effect on increasing the HDT of the unreinforced blend. In combination, these co-compatibilizers produce an interpenetrating network by quenching of the mixed system to a temperature at which demixing is not kinetically favored, without raising the HDT of the blend above the HDT of PVC.
It is this characteristic of the blend co-compatibilized with both CPE and PO-G which results in its excellent resistance to solvents, fatty acids, and ultraviolet light degradation, acceptable low temperature impact strength, paintability and printability, all of which characteristics make articles of the polyblend economically most attractive. The fact that the HDT of the blend is no better than that of PVC or PP-G is not a sufficient detriment to outweigh its processability and other properties particularly useful in a food container.
The GFR polyblend of this invention, in particular is of commercial significance because its excellent mechanical compatibility is obtained with readily available bulk polymers, making the polyblend inexpensive; and, because the GFR polyblend is easily thermoformable to provide substantially rigid shaped articles; yet the GFR polyblend has a higher HDT than one would expect of a GFR predominantly PVC polyblend containing CPE, irrespective of whether the other components were PP or PE, or PP-G. In addition to the properties of the polyblend mentioned hereinabove, GFR articles made from the inexpensive polyblend have durability over a wide range of temperature, making the articles highly marketable.
The GFR polyblend is especially suited for thermoforming sheets of it into large shaped articles such as rocker panels on automobile bodies, but may be used for smaller articles such as instrument panels, dash boards and window trim in vehicles, boats and airplanes. GFR polyblend with long glass fibers, in the form of pellets, may be injection molded to produce pump housings for solvent pumps and for acid service at moderate temperatures.